The invention relates to optical encoders. Specifically, the invention relates to a Moirxc3xa9 type optical encoder for use in a control system.
Incremental optical encoders are well known devices used to track the relative position and movement of an object along a particular track. Typical optical encoders include a light source that emits a light beam, a modulation means (usually a reticle or grating) for modulating the light beam as the object moves along the track, and a detector assembly for receiving the modulated light beams and converting the optical signal into an electrical signal. Multiple detectors may be used obtain two electrical signals that have a constant phase relationship. Together, the two electrical signals indicate both the change in location and the direction of movement of the object.
A specific type of optical encoder, known as a Moirxc3xa9 type encoder, uses two periodic gratings or reticles to modulate the incoming light signal. A typical Moirxc3xa9 encoder construction is depicted in FIG. 1. Referring to FIG. 1, the light source 21 illuminates the scanning reticle 20, generating a periodic radiation pattern. Light that permeates the scanning reticle 20 impinges on the object reticle 30 and light sensors 23 and 24 detect light transmitted through the object reticle 30. As the scanning reticle 20 and the object reticle 30 are translated with respect to one another along the axis indicated by arrows 31, the intensity pattern (not shown) at the surface of the sensors 23 and 24 varies periodically. This periodic variation of intensity, known as a Moirxc3xa9 pattern, is dependent on the spatial periodicity of the object reticle 30 and the scanning reticle 20. Conventionally, sensors 23 and 24 are positioned or oriented with respect to the object reticle 30 and to one another, such that the optical signals that they receive have a constant spatial phase difference of xc2xc of the Moirxc3xa9 period. Since the optical signals received by the sensors 23 and 24 are phase shifted, the electrical signals (not shown) produced by the two sensors 23 and 24 are also phase-shifted from one another by xc2xc period.
The functionality of a Moirxc3xa9 type encoder is shown in FIG. 2. FIG. 2-A shows a magnified view of a portion of an object reticle 30. The object reticle 30 has a periodic pattern of apertures 32 and opaque portions 33 extending in the y direction. The period (or pitch) of the object reticle 30 is labelled Ly. For ease of reference, the period Ly of a reticle 30 is referred to throughout this application as the xe2x80x9cpitchxe2x80x9d. Any one individual pitch Ly including both the aperture 32 and the opaque area 33 is referred to in this application as a xe2x80x9ccellxe2x80x9d of the reticle. The quantity ly/Ly represents the fraction of a cell that is occupied by the aperture and is referred to throughout this application as the xe2x80x9caperture duty cyclexe2x80x9d.
As the object reticle 30 and the scanning reticle 20 (see FIG. 1) are scanned in the y direction relative to one another, an optical signal is received at each of the two sensors 23 and 24. The signals A and B of FIG. 2-B are idealized representations of the signals produced on sensors 23 and 24 respectively. The plot of signals A and B depicted in FIG. 2-B shows the variation of light intensity measured on the sensors 23 and 24 as a function of the relative movement between the object reticle 30 and the scanning reticle 20 in the y direction. It will be appreciated from the plot in FIG. 2-B, that signals A and B are phase separated by xc2xc period.
Assuming an intensity of Io is measured on signal B, the relative position of the object reticle 30 could be yo or yoxe2x80x2. As a result, typical Moirxc3xa9 encoders measure a second signal A to distinguish between the two possible positions yo and yoxe2x80x2. For example, if signal B is measured at Io and signal A is measured at I1, then the system knows that the correct position is yo rather than yoxe2x80x2. In most circumstances, a Moirxc3xa9 system can determine the direction of relative motion by measuring either one of signals A or B. For example, if yo is the start position, then movement in one direction will cause an increase in the intensity of signal B and movement in the other direction will cause a decrease in the intensity of signal B. Hence, if an increase or decrease in the intensity of signal B is detected, then the direction of motion is known. In some circumstances, however, signal B will be at or near a zero derivative point (i.e. at a maximum or minimum of the signals, such as y1, which is a minimum of signal B). In such a situation, both directions of movement will produce similarly increasing intensity profiles for signal B. Signal B is said to be xe2x80x9cindeterminatexe2x80x9d as to direction; consequently, signal A must be used to determine the direction of motion. With two signals (A and B) differing in phase by a known phase difference, such as xc2xc of the Moirxc3xa9 period, at least one signal will always be determinative of the direction of motion.
The principal drawback with incremental encoders, such as the one described above, is that they are only useful for determining relative position and movement. That is, they are only able to determine the position and movement of an object relative to a fixed or predetermined reference position. Often, the reference position used is the start position of the device when the encoder is powered up. Other techniques for obtaining a reference position include using an index signal that alerts the encoder system when the object is at a particular position along its track. This requires that, upon xe2x80x9cwake-upxe2x80x9d, the encoder searches its track for the index signal, before it is able to locate itself. The dependence of incremental encoders on a reference position is an obvious drawback in some applications, where the start position may not be suitable for a reference, where the provision of an index signal is inconvenient or impossible, or where the time required to locate an index signal is not available.
Some optical position encoders, which do not rely on a reference position are known in the art and are referred to as xe2x80x9cabsolute positionxe2x80x9d encoders. A typical implementation for an absolute position encoder is depicted in FIG. 3. The encoder includes a light source 11, such as an LED, for emitting light La and a collimating lens 12 to produce collimated light Lb. A first scale 13 is a specialized grating with a number of grating tracks (t1, t2, . . . tn), each track including apertures 13A and opaque sections 13B. For each track (t1, t2, . . . tn), the apertures 13A and the opaque sections 13B alternate periodically. However, although the aperture duty cycle is constant for each track (t1, t2, . . . tn), the pitch of each track (t1, t2, . . . tn) is different. A second scale 14 is provided with apertures (14 A1, 14A2 . . . 14An) arranged behind the respective grating tracks (t1, t2, . . . tn). The arrangement of the second scale 14 is such that light transmitted through the apertures 13A of the first scale 13 is able to pass through the apertures (14A1, 14A2 . . . 14An). Photodetectors (15-1, 15-2, . . . 15-n) are positioned strategically with respect to the apertures (14A1, 14A2 . . . 14An), so as to convert the light beams passing through the apertures (14A1, 14A2 . . . 14An) into electrical signals.
Typically, the prior art absolute position encoders use a first scale 13, which is provided with binary xe2x80x9cGray codesxe2x80x9d as shown in FIG. 4, wherein grating pitches (P1, P2, . . . Pn) between adjacent grating tracks (t1, t2, . . . tn) have a ratio of 1:2. Consequently, the intensities of the light beams (Le1, Le2, . . . Len) received by the respective photodetectors (15-1, 15-2, . . . 15-n) change periodically when the first scale 13 moves in a longitudinal direction (marked by arrow m). Similarly, the electrical signals (S1, S2, . . . Sn) produced by photodetectors (15-1, 15-2, . . . 15-n) also change periodically as is depicted in the graph of FIG. 5. FIG. 5 depicts the electrical signals (S1, S2, . . . Sn) on the vertical axis as a function of displacement along the longitudinal axis (marked by arrow m) on the horizontal axis. FIG. 6 then shows a block diagram of how the electrical signals (S1, S2, . . . Sn) are digitized by individual comparators 50 into digital signals (d1, d2, . . . dn) and further converted from the binary Gray codes into an absolute positional data D by the decoder 51. The absolute position data D could be a simple binary code, a BCD code or some other representative scheme.
One drawback with this type of encoder that employs a Gray code or similar encoding scheme is that the resolutional detection is limited by the grating pitch Pa in the track ta. Detection of smaller increments is impossible. Also, the detection stroke or largest measurements that can be made are limited to the order of the grating pitch P1 in track t1. Any attempt to expand the performance to facilitate larger or smaller positional measurements increases the number of grating tracks, thereby increasing the size of the device and the number of components such as photodetectors and comparators obviously, increasing the size of the encoder and increasing the number of components to achieve a larger measurement range limits its possible design applications and increases the cost and complexity of the device.
A second limitation of this type of encoder that employs a Gray code or similar encoding scheme is the impracticality of expanding it to function as a two dimensional encoder. As depicted in FIG. 3 above, measurement of a particular dimension requires a first scale 13 with a large number of grating tracks (t1, t2, . . . tn), which extend in a second orthogonal dimension. As mentioned above, any attempt to expand the performance to facilitate larger or smaller positional measurements increases the number of grating tracks, thereby increasing the size of the device in the second orthogonal dimension. Because of the increase in size on the second orthogonal dimension, this size issue becomes even more of an impediment when the device is used to attempt to implement a two dimensional encoder. Clearly, the expansion in size presents extra difficulties to design applications when trying to use this type of encoder to measure absolute position in two dimensions.
Other types of absolute position encoders that depend on diffraction and the wave nature of light are known in the art. Such encoders utilize a grating with a varying pitch to produce a number of diffracted orders. A plurality of detectors convert the intensity of the diffracted orders into electronic signals for analysis and extraction of positional information. The need to detect a number of diffracted orders individually and the corresponding space occupied would make it difficult to extend the system to a two dimensional case. As a result, these types of encoders may not be suitable for implementation in some applications, particularly where space is at a premium.
In the field of three-dimensional Moirxc3xa9 shape analysis or Moirxc3xa9 topography there are several disclosures of systems which project a grid pattern onto an object to be inspected and create a Moirxc3xa9 interference between the light reflected from the object and a second reference grid. In these inventions the concern is primarily with determining a deflection or the topography of an object where the motion or information to be determined is substantially in the direction of the projected beam and hence not useful for use in a two-dimensional encoder.
There is a need for an optical encoder that ameliorates at least some of the disadvantages of the prior art systems mentioned above.
In accordance with the present invention a structured radiation source is used to project a pattern on a surface or reticle which is also patterned. The patterns are selected to vary in some manner which enables an absolute position to be determined by detecting the changes in the radiation pattern either reflected from or transmitted through the surface.
The patterning can be in the form of a plurality of cells which have either transmitting and non-transmitting or reflecting and non-reflecting portions. Advantageously the surface or reticle can be made up of a grid of lines varying in thickness while maintaining a fixed pitch or spacing in two-dimensions.
There are also certain advantages, as will be described in the preferred embodiment, in using reticles with concentric circles, or a grid where lines in different orthogonal directions have sensitivity to different wavelength or polarization of radiation .
Advantageously the radiation source can be realized through a plurality of individual radiation sources which can be driven to project patterns onto the reticle. A convenient radiation source is a radiation emitting diode or light emitting diode.
The radiation source may comprise a matrix of radiation emitting devices which can be driven to produce lines of radiation in two orthogonal directions.
The radiation sources are projected using a lens or some other means onto the reticle surface where the Moirxc3xa9 interference is generated and detected by a radiation sensitive device which could be a photodetector or even a CCD sensor.
Multiple encoder units can be constructed for each radiation source which allows the running of many such encoders in parallel while minimizing the space required to house the devices.
Through careful choice of the geometry of the radiation sources, it is possible to implement a method of measuring position in two places on the reticle and then determining a rotation angle which may be necessary for achieving the best accuracy of position encoding.
This invention is also particularly suited to an application, such as a control system, where it is necessary to servo a position in two dimensions employing the encoder to measure the actual position.
These and other objects of the present invention will be better understood from the following more detailed description along with the drawings and the accompanying claims.